The present invention relates to a steering control system for tracklaying vehicles such as, for instance, bulldozers.
There is known a steering control system for tracklaying vehicles such as bulldozers, in which a clutch and a brake are provided for each of the right and left driving wheels of the vehicle and controlled to make a right or left turn of the vehicle. In this steering control system, the steering lever is operated in a right or left steering direction during travel of the vehicle to release the clutch on the side, to which the steering lever has been operated, from its engaged state while the brake on the same side being actuated in a braking direction so that only the crawler belt on this side stops, whereby the vehicle turns in a desired direction.
The above-described steering control system is usually designed such that a specified period of time elapses after the clutch has been released until the brake is actuated, or alternatively, such that a specified period of time elapses after the brake has been released until the clutch is engaged, whereby shocks occurring during the steering control are mitigated to provide a smooth turn.
Such an arrangement, however, has presented the problem that if hydraulic pressure characteristics such as the time interval between clutch disengagement and brake engagement or between brake disengagement and clutch engagement, the hydraulic pressure at the start of brake engagement and the hydraulic pressure at the start of clutch disengagement, are made to be constant at all times, the optimum clutch and brake control for the respective bulldozer""s operation states (e.g., dozing, slope traveling and operation at low engine rotational speed) cannot be performed.
As an attempt to solve the above problem, the present applicant has previously proposed a steering control system for tracklaying vehicles in Japanese Patent Application No. 2000-177618 according to which, with the aim of achieving steering performance optimum for each of the various operating conditions of a tracklaying vehicle, the clutch/brake modulation characteristic is varied according to, for example, xe2x80x9chigh-load drive statexe2x80x9d, xe2x80x9cslope travel statexe2x80x9d, xe2x80x9clow engine rotational speed statexe2x80x9d and xe2x80x9chigh-speed travel statexe2x80x9d.
In the steering control system disclosed in the prior application, there are still remained the following drawbacks to be remedied.
(1) If the vehicle tries to make a turn during ascent travel, disengagement of the clutch on the side to which the steering lever has been operated causes the crawler belt on the same side to move, descending the slope because of its own weight. This leads to the so-called counterrotation phenomenon, that is, a turn of the vehicle on the spot in the direction in which the steering lever has been operated.
(2) Since the control for preventing a turn in a reverse direction (i.e., reverse steering) during the vehicle""s descent travel has priority over the control in the dozing operation state (forward drive with a great tractive force being exerted), if dozing operation is carried out during descent travel, turning hydraulic pressure becomes high at the time of a turn during the dozing operation, resulting in an abrupt turn.
(3) The break hydraulic pressure required for a turning movement becomes high when the engine comes into its low rotational speed state during descent travel of the vehicle. Therefore, if the control for normal descent travel is performed, a shock will occur while the vehicle is turning.
(4) When a low speed range is selected during travel of the vehicle, turn control ability deteriorates owing to the change in brake hydraulic pressure.
The present invention is directed to overcoming the foregoing problems and a primary object of the invention is therefore to provide a steering control system for a tracklaying vehicle, which provides smooth turning performance according to various modes such as an ascent travel mode, descent travel mode and low speed range travel mode and which provides improved controllability.
The above object can be accomplished by a first aspect of the invention according to which, there is provided a steering control system for a tracklaying vehicle, which has a clutch and brake provided for each of right and left driving wheels, electronic proportional control valves for controlling the clutches and brakes, a steering lever, a steering command signal generator for generating a steering command signal according to the operation amount of the steering lever, and a controller for issuing a steering control signal to one of the electronic proportional control valves in response to an output from the steering command signal generator,
which further comprises pitch angle detecting means for detecting the pitch angle of the vehicle inclining back and forth, and
wherein if it is determined by the pitch angle detecting means that the vehicle is in an ascent travel state, the controller outputs a steering control signal to one of the electronic proportional control valves to obtain a hydraulic pressure characteristic which makes the range of a clutch holding zone wider than that employed when the vehicle is in a flat ground travel state.
According to the first aspect of the invention, the range of the clutch holding zone during ascent travel is made to be wider than that employed during flat ground travel and, in accordance with this, the range of the brake holding zone is narrowed, so that the hydraulic pressure zone at the time of turning can be shifted to the side of the clutch holding zone. Accordingly, the value of the turning torque for the inner crawler belt at the time of turning becomes plus so that a half-clutch state continues for a long time, preventing an occurrence of the counterrotation phenomenon caused by disengagement of the clutch for the inner crawler belt. As a result, improved turning controllability can be achieved.
Preferably, the invention is arranged such that, if it is determined by the pitch angle detecting means that the vehicle is in its ascent travel state, the controller outputs a steering control signal to one of the electronic proportional control valves to obtain a hydraulic pressure characteristic which makes the lower limit of hydraulic pressure at the time of clutch disengagement lower than that employed when the vehicle is in the flat ground travel state. Since the lower limit of hydraulic pressure at the time of clutch disengagement, that is, clutch disengagement starting hydraulic pressure becomes low when the vehicle ascends, improved turning controllability can be achieved by using a hydraulic pressure characteristic suited for the low lower limit of hydraulic pressure.
Preferably, in the invention, a plurality of kinds of characteristic diagrams, each of which step-wise varies for every hydraulic pressure characteristic, are prepared and a desired characteristic diagram is selected from them. By virtue of this arrangement, a turning characteristic suited for each condition can be selected so that the optimum control performance can be constantly achieved.
According to a second aspect of the invention, there is provided a steering control system for a tracklaying vehicle, which has a clutch and brake provided for each of right and left driving wheels, electronic proportional control valves for controlling the clutches and brakes, a steering lever, a steering command signal generator for generating a steering command signal according to the operation amount of the steering lever, and a controller for issuing a steering control signal to one of the electronic proportional control valves in response to an output from the steering command signal generator,
which further comprises tractive force detecting means for detecting the tractive force of the vehicle and pitch angle detecting means for detecting the pitch angle of the vehicle inclining back and forth, and
wherein if it is determined by the tractive force detecting means and the pitch angle detecting means that the vehicle is not in a high-load drive state but in a descent travel state, the controller outputs a steering control signal to one of the electronic proportional control valves to obtain a hydraulic pressure characteristic which eliminates a zone in which a clutch and a brake are released at the same time and sets the upper limit of hydraulic pressure at the time of brake engagement to a first value, the first value being lower than the upper limit of hydraulic pressure when the vehicle is in a flat ground travel state.
According to the second aspect of the invention, when the vehicle is in a descent travel state, control is effected such that there is no zone where a clutch and a brake are released at the same time, in other words, the braking force of the brake is increased while the clutch disengagement zone being eliminated, so that the reverse-directional turning (reverse steering) phenomenon can be positively avoided during descent travel by disengagement of the clutch and shock-free turning performance can ensured. In addition, since a dozing steering mode has priority over a descent steering mode, an abrupt turn of the vehicle can be prevented, this abrupt turn being caused by an increase in turning hydraulic pressure when the vehicle turns for dozing operation during descent travel.
Preferably, the steering control system of the invention further comprises rotation detecting means for detecting the rotation of an engine, and if it is determined by the rotation detecting means that the engine is in its low rotational speed state, the controller outputs a steering control signal to one of the electronic proportional control valves to obtain a hydraulic pressure characteristic which sets the upper limit of hydraulic pressure at the time of brake engagement to a second value, the second value being lower than the upper limit of hydraulic pressure when the vehicle is in the flat ground travel state and higher than the first value. With this arrangement, even if the rotational speed of the engine drops during descent travel, the upper limit of hydraulic pressure at the time of brake engagement is corrected to a higher value than the upper limit of hydraulic pressure during normal descent travel, reducing breaking force, so that a shock caused by a turn is mitigated and turning controllability is improved.
In the invention, it is preferable to prepare a plurality of characteristic diagrams each of which stepwise varies for every hydraulic pressure characteristic and to select a desired characteristic diagram from them. This enables selection of a turning characteristic suited for each condition to constantly obtain the optimum control performance.
According to a third aspect of the invention, there is provided a steering control system for a tracklaying vehicle, which has a clutch and brake provided for each of right and left driving wheels, electronic proportional control valves for controlling the clutches and brakes, a steering lever, a steering command signal generator for generating a steering command signal according to the operation amount of the steering lever, and a controller for issuing a steering control signal to one of the electronic proportional control valves in response to an output from the steering command signal generator,
which further comprises speed range detecting means for detecting the speed range of a transmission, and
wherein if it is determined by the speed range detecting means that the transmission is placed in a low speed range, the controller outputs a steering control signal to one of the electronic proportional control valves to obtain a hydraulic pressure characteristic which sets the rate of change of hydraulic pressure with respect to lever stroke when hydraulic pressure changes from its upper limit to its lower limit at the time of brake engagement to a low value.
According to the third aspect of the invention, since control is made such that the rate of change of hydraulic pressure for lever stroke when hydraulic pressure changes from its upper limit to its lower limit during brake engagement decreases as the transmission is shifted to a lower speed range, the effective lever stroke zone when the transmission is placed in a low speed range can be expanded to achieve smooth turning performance.
In the invention, it is preferable to set the rate of change of hydraulic pressure to a low value by setting the lower limit of hydraulic pressure when the transmission is placed in a low speed range to a value higher than the lower limit of hydraulic pressure when the transmission is in a high speed range.
In the invention, it is also preferable to prepare a plurality of characteristic diagrams each of which stepwise varies for every hydraulic pressure characteristic and to select a desired characteristic diagram from them. This enables selection of a turning characteristic suited for each condition to constantly obtain the optimum control performance.